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Yes those caps do come off as it turns out. I'm a bit pssd off with myself for not checking this out properly a while ago - I saw the label saying maintenance free and assumed that they couldn't be standard lead acids so I parked it until now while I got on with getting to grips with the rest of the boat's systems. Not a good idea as it turns out! Ah well, chalk it up to experience I guess.

So I'll give them a clean, top up the cells with deionised water charge them up as much as I can and then get the Mastervolt to do an equalisation charge. It's a 12V system so I take it I should simply halve the values given in Dave's post (so 14.8V absorption and 13.8V float) and set the Outback accordingly. If equalisation freshens them up then we'll then look at nursing them through the winter and buying replacements next year but if it doesn't we might see if we can bring that forwards.

Sorry, yeah I always think in 24v. All it means is the batteries are on the way out, you need to run em a bit hotter, so they see it through till morning, they will use more water being run at a slightly higher voltage and gas a bit more, so need good ventilation and topping up more often, put a light directly over them so you can get a good look inside for topping up.The traction cells I have recently bought are made by eternity, the ones we have at home in wales were made by chloride about 15-20 years ago, depending on how old they were before we bought them, we have had them nearly 10 years, they are elderly but keep soldering on.

I have wrecked plenty of those Varta batteries Scatter, they're great for use in boats that are cruising but won't last a year cycled deeply and not charged fully. Trojan's are fine too, I look after a bank of those and they're doing OK after three years. The Rolls are what I plan to buy for my new house after seeing how well they preform at an 'off grid' property nearby. There's nothing wrong with forklift cells, it's what I use at the moment and have been doing so for seven years. However I feel that they are just the wrong shape, especially for a boat, they are after all designed to act as a counterweight as well as an energy storage medium.

I haven't done a formal power audit but know that our demand is pretty low (I work in energy efficiency so aim to cut my energy demand as far as possible). LED lighting throughout, very efficient fridge and TV, a laptop and that's about it. We're averaging about 0.6kWh generation per day at the moment and got 1.1kWh today. That really should come close to covering our demand I think but I should sit down and add it all up to make sure.

"Peliminary results agree well with established general understanding that the charge efficiency of flooded lead-antimony batteries declines with increasing state-of-charge, and that charge efficiency is a non-linear function of battery state-of-charge. These tests indicate that from zero SOC to 84% OC the average overall battery charging efficiency is 91%, and that the incremental battery charging efficiency from 79% to 84% is only 55%. This is particularly significant in PV systems where the designer expects the batteries to normally operate at SOC above 80%, with deeper discharge only occurring during periods of extended bad weather. In such systems, the low charge efficiency at high SOC may result in a substantial reduction in actual available stored energy because nearly half the available energy is serving losses rather than charging the battery. "..."Charge efficiencies at 90% SOC and greater were measured at less than 50% for the battery tested here, requiring a PV array that supplies more than twice the energy that the load consumes for a full recovery charge. Many batteries in PV systems never reach a full state of charge, resulting in a slow battery capacity loss from stratification and sulfation over the life of the battery."

If the batteries are in poor shape then their charge efficiency will be even worse. However from what you've said it doesn't sound like your batteries spend much time above 79% SOC so perhaps the low incremental charge efficiency at higher states of charge won't be much of an issue....

I couldn't find any efficiency graphs for the inverter, but it is specified at >= 90% efficiency above 100VA. I guess your typical load is quite low so I would expect the efficiency to be around 90% at loads around 100VA, dropping sharply for lower loads. Efficiency might drop for loads with a poor power factor as well, but the best way to determine your actual power use would be to measure the current and voltage going into the Mastervolt.

I see that the inverter consumes between .5 and 9W depending on the mode - 9W is 216Wh/day so a big chunk out of .6kWh/day generation. If you can't live with the inverter in its lowest power mode (which switches on every 2.5 secs to check for a load), it may be worth trying a low power (say 100W or so?) inverter in parallel to supply low loads and operating the Mastervolt in its lowest power mode. Whether you can get a small inverter with acceptable efficiency is another matter and there's also the issue of the stability of the system when the load increases to the point when the Mastervolt decides to kick in. You'd have to experiment and so how it behaves.

The FM80 charger manual includes efficiency curves for different input voltages - looks like you could easily get 5% better efficiency by having the panels in parallel if you haven't already done so. Might also be worth experimenting with?

Cheers Splyn, most helpful. I was idly wondering what the round trip efficiency was - not particularly encouraging for winter charging from PV!

I topped up the cells and tried to do an equalisation charge once the Mastervolt was indicating it was in float mode. I flick the dip switch and the five indicator lights flash as they should to indicate equalisation mode but it drops out of equalisation mode after just a few minutes. I've tried twice and no joy

When it was in equalisation mode the voltage was varying between about 13.4V and 13.9V, certainly not the gradual increase up towards 15.5V indicated in the manual.

Its actually going to be worse than the Sandia labs test results show - the efficiency graphs are the coulometric efficiency (ampere hours out / ampere hours in) not energy efficiency. The difference in charge and discharge voltage will cost another 5 to 10% of those precious kWhs that charge the batteries.

If the batteries are cold in the winter then another few % could be lost on top.